You can indeed do this with Sleep as Android - I use it in combination with a Pebble Time to feed Sleep as Android accelerometer data (for sleep phases), and th

You can indeed do this with Sleep as Android - I use it in combination with a Pebble Time to feed Sleep as Android accelerometer data (for sleep phases), and then the watch vibrates for an alarm - falling back to an audible alarm on the phone if I don't deactivate it in time. The only downside is that the vibration motor in the Pebble Time is a little bit loud, but still much quieter than an audible alarm on the phone itself.